@Phgerin I have contacts on my mac's Address Book that have birthday entries. I have sync'd my mac with my phone and the N900 reproduces the birthdays in the contact's information faithfully. These in turn show up on the Birthdays calendar on the N900. I can also edit the birthday (or add it in the first place) on the N900 contact edit screen. Save the contact and sync it and the entry in Address Book reflects the change correctly. Are you still experiencing that issue?